ACIS Observations taken at -110 C
The ACIS focal plane temperature was lowered from -110 C (163 K) to -120 C (153 K) on 2000 January 29. While the CIAO threads are applicable to both temperatures, this document highlights details particular to analyzing data taken at -110 C.
The focal plane temperature and observation date are recorded in the header of the event file:
unix% dmkeypar acis_evt1.fits FP_TEMP echo+ 163.952042 unix% dmkeypar acis_evt1.fits DATE-OBS echo+ 1999-12-31T03:22:39
There are gain files for -110 C observations, which are used to compute the PHA, ENERGY and PI values reported for the events. However:
there is no CTI Correction available
there is TGAIN calibration data for the ACIS-S1 (ccd_id=5) and S3 (ccd_id=7) back-illuminated chips only
Reprocessing the Data
The TGAIN calibration files for -110 C were released for the first time in CALDB 3.4.3 (31 March 2008).
The detector name and CALDB version used in standard data processing are recorded in the header of the event file:
unix% dmkeypar acis_evt1.fits DETNAM echo+ ACIS-456789 unix% dmkeypar acis_evt1.fits CALDBVER echo+ 1.6
Since this is an older observation which uses both the S1 and S3 chips, it should be reprocessed to apply the TGAIN correction. Run the chandra_repro script or follow the Create a New Level=2 Event File thread apply the new calibration.
The tool will print a warning because the CTI calibration data does not exist for -110 C data:
unix% acis_process_events Input event file or stack (acis_evt1.fits): Output event file name (acis_new_evt1.fits): aspect offset file ( NONE | none | <filename>) (pcad_asol1.fits): # acis_process_events (CIAO 4.4): WARNING: problem reading ctifile, cti adjustment will not be applied. Changing apply_cti=yes to apply_cti=no.
This is only a warning; the tool still produces a usable level=1 event file. Complete the rest of the thread to create a new level=2 event file.
Imaging spectroscopy: creating RMFs
Users have to choose which RMF tool to use based on which chip(s) are used in the analysis. Using the wrong tool will result in mismatched calibration and unreliable results.
- Back-illuminated chips S1 and S3, with the TGAIN correction: use mkacisrmf.
- Back-illuminated chips S1 and S3, without the TGAIN correction: use mkrmf.
- Front-illuminated chips (I0-3, S0, S2, S4-5): always use mkrmf
For more information, refer to the mkacisrmf why topic.